Precision rotary actuators are used in applications where there is a requirement to rotate a shaft through a precisely defined angular increment. For example, an electronic camera having full automatic capability may require a rotary actuator to adjust the f-stop, another to advance the film, and a third to focus the lens. Conventional stepping motors are frequently used to provide the precise incremental rotary motion required for applications such as this. The available power for such motors is usually limited, requiring that the motor be constructed with very efficient high impedance windings to ensure that minimal battery power is consumed. These considerations are not limited to stepping motors, but also often apply to applications wherein a continuously rotating motor is required.
As an alternative to electromagnetic motors, engineers have turned to piezoelectric technology to develop motors without windings that do not use a magnetic field to develop a rotational torque but, instead, are driven by the expansion and contraction of one or more piezoelectric elements to which an electric potential is applied. A multitude of designs for piezoelectric motors driven by an AC voltage is disclosed in U.S. Pat. No. 4,019,073. All the motors shown in this patent include a rotor and a stator, at least one of which comprise a piezoelectric vibrator for inducing rotational motion in the rotor.
Another piezoelectrically-driven motor design is disclosed in U.S. Pat. No. 4,468,583. In this design, the motor also includes a rotor and a stator. A pair of annular piezoelectric elements disposed on each side of a holding member on the stator contract and expand in the radial direction in response to an applied electrical signal, and act to alternately clamp and release the rotor. A plurality of other piezoelectric elements are disposed between the annular elements and the holding member of the stator, and are operative to rotate the rotor by causing a displacement of the annular elements in the circumferential direction, while the annular elements are clamped on the rotor.
Most of the piezoelectric motors described in the prior art apply a driving torque to rotate a single drive shaft. However, higher output torque may be achieved with a piezoelectric motor having dual counterrotating drive shafts. The dual shafts may be adapted to drive a single output shaft on which is mounted a gear commonly driven by the dual drive shafts, thereby substantially increasing the output torque of the motor. Dual counterrotating shaft also serve to minimize or eliminate reaction torque produced by rotation of a single drive shaft. Furthermore, selective engagement of one of the dual drive shafts can enable quick reversal of a driven mechanism from one direction of rotation to another. These and other advantages of a dual drive shaft motor are well known in the art, but are not efficiently implemented by prior piezoelectric motors.